SQL backend?

Wandering through OpenLDAP 2.1's docs i've seen the possibility of using a
so-called "SQL programmable backend" -- nothing else on the topic.
Could somebody possibly give me some pointers to where I can find
additional information? Precisely, about using MySQL as a backend for OpenLDAP.

I'm very interested in accessing data contained in a DB through LDAP, and
wouldn't mind coding if needed, just wanted to know before "reinventing the
wheel", as well as some comments, advice,... whatever!